1. Field of the Invention
This disclosure relates to optical amplifiers, and, in particular to wavelength-stabilized pump diodes for pumping gain media in an ultrashort (e.g., chirped) pulsed laser system.
2. Description of the Prior Art
FIG. 1 illustrates an optical amplifier 100 in accordance with the prior art. The optical amplifier 100 includes an actively wavelength-stabilized pump 110, a combiner 120, and a gain fiber 130. The pump 110 provides a continuous-wave pumping light at a relatively constant wavelength that is coupled by the combiner 120 into the gain fiber 130. The gain fiber 130 is energized by the pumping light and thereby amplifies a laser signal 140 directed through the combiner 120 into the gain fiber 130. The gain fiber 130 fiber typically yields small-signal gains of commonly around 10 dB to 35 dB for the optical amplifier 100.
To produce the pumping light, the pump 110 comprises one or more pump diodes 150 coupled by a single-mode optical fiber to a fiber Bragg grating 155. The fiber Bragg grating 155 is slightly reflective (e.g., 1%–3%) so that a small amount of pumping light generated by the pump diodes 150 at the appropriate wavelength is reflected back into the pump diodes 150. The pump diodes 150 enter a coherence collapse regime, whereby instead of emitting a single laser spectral line that mode hops (i.e. changes wavelength) and fluctuates in power at the output of the pump diodes 150, the pumping light is a relatively stable wavelength with a shaped characteristic (e.g., square, rounded, etc.) due to the fiber Bragg grating 155.
A limitation with the optical amplifier 100 is that the pump diodes 150 generally drift in wavelength with temperature and drive current. Because the peak absorption of the gain fiber 130 may be fairly narrow spectrally, if the pump diodes 150 drift slightly off-wavelength by as little as a few nanometers, then the gain and output power of the optical amplifier 100 is reduced. The resulting drift or shift in wavelength of the output of the pump diodes 150 significantly reduces output power and power efficiency of the optical amplifier 100. If one wishes to excite the gain fiber 130 at a wavelength that does not correspond to its peak absorption, then one must use longer lengths of gain fiber 130 to achieve similar gain in the optical amplifier 100 as when the peak absorption of the gain fiber 130 is excited by the pump diodes 150.
Therefore, in the conventional optical amplifier 100, temperature of the pump diodes 150 must be maintained within a few degrees of a desired operating temperature in order for the wavelength of the pumping light to remain fairly stable. For example, with a temperature coefficient of drift of the pump diodes 150 of typically 0.3 nm/degree Kelvin (K), then if the temperature of the pump diodes 150 is allowed to drift by 10 degrees then the wavelength of the pumping light from the pump 110 may drift by 3 nm.
Consequently, to improve temperature and current stability and reduce wavelength drift of the pumping light, the pump 110 conventionally includes a temperature and/or current control 160. The temperature and/or current control 160 may include a water chiller and heat exchanger or thermoelectric converters (TEC elements or Peltier devices) for keeping the pump diodes 150 at a fairly constant temperature.
One limitation with water chillers is that chillers are relatively large bulky devices that increase the size and power draw of the optical amplifier 100. TEC elements are generally very expensive, relatively inefficient, and consume large amounts of power. TEC elements typically consume three times more energy than the amount of heat energy to be removed from the pump diodes 150. For example, to remove 5 watts of heat energy from the pump diodes 150, the TEC elements may require 15 watts of power.
The pump diodes 150 themselves are generally not highly efficient, typically converting only 25–50% of consumed power to pumping light output. For example, with 100 watts of electricity into the pump diodes 150, only about 28 watts of pumping light may be generated by the pump diodes 150. Furthermore, the gain fiber 130 has a relatively low conversion efficiency, with typically only 20% of the light input converted to the desired wavelength.
Therefore, including other inefficiencies in the system of which the optical amplifier 100 is merely one component, overall efficiency of the system can be as low as 1%. For example, for a 10 watt laser light output, the system may require 1 kilowatt of electrical power input. The remaining 990 watts of energy is converted into heat, which is a very inefficient conversion of electrical energy to useful laser light output from the system. The overall power-to-light conversion efficiency (also referred to as power or plug efficiency) of the optical amplifier 100 is relatively low, and the plug efficiency of the optical amplifier 100 is further reduced by the temperature and/or current control 160.